The use of debonders and softening agents in facial and bath tissue is a common practice in the tissue industry because of consumer demand for soft products. However, even though the level of softness has increased generally over the years, industry efforts continue in order to provide further improvements in softness for facial and bath tissues.
It has been discovered that the perceived softness of tissue products can be improved by producing a tissue structure containing two different chemical softening additives, namely an imidazolinium quaternary compound and a cationic amidoamine compound. With proper formation and creping, the synergistic combination of these two different softening additives results in a unique tissue structure that is surprisingly soft. More specifically, the tissues of this invention have a combination of low internal density, a high formation index and a high surface smoothness.
Hence in one aspect, the invention resides in a soft tissue comprising from about 0.01 to about 1 dry weight percent of a imidazolinium quaternary compound and from about 0.01 to about 1 dry weight percent of a cationic amidoamine compound, said tissue having a Formation Index of 160 or greater, a Void Volume of 9.0 grams per gram or greater and a crepe structure, as measured by the Optical Surface Crepe method, having 60 or fewer crepe peaks having a peak height of 93 microns or greater.
In another aspect, the invention resides in a method of making a soft tissue comprising the steps of: (a) adding an imidazolinium quaternary compound to an aqueous suspension of papermaking fibers; (b) depositing the papermaking fibers onto a papermaking fabric to form a wet web; (c) partially dewatering the wet web; (d) pressing the dewatered web against the surface of a Yankee dryer, wherein the surface of the Yankee dryer contains a creping adhesive and a cationic amidoamine; and (e) drying the web and creping the dried web from the surface of the Yankee dryer such that the resulting tissue has a Formation Index of 160 or greater, a Void Volume of 9.0 grams per gram or greater and a crepe structure, as measured by the Optical Surface Crepe method, having 60 or fewer crepe peaks having a peak height of 93 microns or greater.
As used herein, imidazolinium quaternary compounds useful for purposes of this invention have the following structure: 
where:
R1=saturated or unsaturated, substituted or unsubstituted, aliphatic hydrocarbon having a carbon chain length of 8 or greater;
R2=C1-4 alkyl; and
Axe2x88x92 any suitable anion including, but not limited to, Clxe2x88x92, Brxe2x88x92, Fxe2x88x92, or R2SO4xe2x88x92.
The imidazolinium quaternary compound is suitably added at the wet end of the tissue making process, such as adding it to the thick stock prior to web formation, where the consistency of the aqueous papermaking fiber suspension is about 2 percent or greater. The imidazolinium quaternary compound can be added to the papermaking fiber suspension of a blended (non-layered) tissue or a layered tissue. If the tissue is to be layered, the imidazolinium quaternary compound can be added to the furnish of the layer that ultimately contacts the creping cylinder surface. In most cases, this will also be the layer that is the outwardly facing layer of the final tissue product, which the consumer contacts.
The amount of the imidazolinium quaternary compound in the finished tissue product can be any amount, more specifically from about 0.01 to about 1 dry weight percent, more specifically from about 0.05 to about 0.5 dry weight percent, based on the dry weight of the fiber in the finished product. Lesser amounts are less effective in providing adequate softness. Greater amounts are effective and can be used, but are less attractive economically.
Suitable imidazolinium quaternary compounds are well known in the art. Suitable commercially available imidazolinium quaternary compounds include: Varisoft 3590 (Witco Corporation); DPSC 5299-8, which is a quaternary imidazolinium blended with a fatty acid alkoxylate and a polyether with a 200-300 molecular weight (Witco Corporation); ProSoft TQ-1003 (Hercules Incorporated); and Mackernium DC-183 (McIntyre Corporation).
In addition to the imidazolinium quaternary compound, nonionic surfactants and/or polyhydroxy compounds can also be added to the tissue at the wet end of the tissue making process to further enhance the softness of the final product. Examples of useful nonionic surfactants and polyhydroxy compounds are disclosed in U.S. Pat. No. 5,730,839 issued Mar. 24, 1998 to Wendt et al., which is hereby incorporated by reference.
The cationic amidoamine compound is suitably added after the tissue has been formed and at least partially dewatered. This can be accomplished by spray application onto the tissue web while supported by a fabric or felt or by application directly to the Yankee dryer by any suitable means such that it is subsequently transferred to the tissue. Suitable commercial cationic amidoamine compounds include Quaker 2008 (Hercules, Incorporated). Other suitable cationic amidoamine compounds include those disclosed by U.S. Pat. No. 6,120,644 to Schroeder et al., which is hereby incorporated by reference.
The amount of the cationic amidoamine compound in the finished product can be from about 0.01 to about 1 dry weight percent, more specifically from about 0.05 to about 0.5 dry weight percent, based on the dry weight of the fiber in the finished product.